The present invention relates to the field of downhole fluid analysis applicable to formation evaluation and testing in the exploration and development of hydrocarbon-producing wells such as oil or gas wells. In particular, the invention provides methods and apparatus suitable for performing downhole analysis on fluids produced in such wells using optical techniques.
In order to evaluate the nature of underground formations surrounding a borehole, it is often desirable to obtain samples of formation fluids from various specific locations in a borehole. Tools have been developed which allow several samples to be taken from the formation in a single logging run. Examples of such tools can be found in U.S. Pat. No. 3,780,575 and U.S. Pat. No. 3,859,851.
The RFT and MDT tools of Schlumberger represent two specific versions of sampling tools. In particular, the MDT tool includes a fluid analysis module to allow analysis of the fluids sampled by the tool. FIG. 1 shows a schematic diagram of such a tool and includes a borehole tool 10 for testing earth formations and analysing the composition of fluids from the formation is shown in FIG. 1. The tool 10 is suspended in the borehole 12 from the lower end of a logging cable 15 that is connected in a conventional fashion to a surface system 18 incorporating appropriate electronics and processing systems for control of the tool. The tool 10 includes an elongated body 19 which encloses the downhole portion of the tool control system 16. The body 19 also carries a selectively extendible fluid admitting assembly 20 (for example as shown in the ""575 and ""851 patents referenced above, and as described in U.S. Pat. No. 4,860,581, incorporated herein by reference) and a selectively extendible anchoring member 21 which are respectively arranged on opposite sides of the body 19. The fluid admitting assembly 20 is equipped for selectively sealing off or isolating portions of the wall of the borehole 12 such that pressure or fluid communication with the adjacent earth formation is established. A fluid analysis module 25 is also included within the tool body 19, through which the obtained fluid flows. The fluid can then be expelled through a port (not shown) back into the borehole, or can be sent to one or more sample chambers 22, 23 for recovery at the surface. Control of the fluid admitting assembly, the fluid analysis section and the flow path to the sample chambers is maintained by the electrical control systems 16, 18.
The OFA, which is a fluid analysis module 25 as found in the MDT mentioned above, determines the identity of the fluids in the MDT flow stream and quantifies the oil and water content. In particular, U.S. Pat. No. 4,994,671 (incorporated herein by reference) describes a borehole apparatus which includes a testing chamber, means for directing a sample of fluid into the chamber, a light source preferably emitting near infrared rays and visible light, a spectral detector, a data base means, and a processing means. Fluids drawn from the formation into the testing chamber are analysed by directing the light at the fluids, detecting the spectrum of the transmitted and/or backscattered light, and processing the information accordingly (and preferably based on the information in the data base relating to different spectra), in order to quantify the amount of water and oil in the fluid. Thus, the formation oil can be properly analysed and quantified by type.
U.S. Pat. No. 5,167,149, and U.S. Pat. No. 5,201,220 (both incorporated by reference herein) describe apparatus for estimating the quantity of gas present in a flow stream. A prism is attached to a window in a flow stream and light is directed through the prism to the window and light reflected from the window/flow interface at certain specific angles is detected to indicate the presence of gas in the flow.
As set forth in U.S. Pat. No. 5,266,800 (incorporated herein by reference), by monitoring optical absorption spectrum of the fluid samples obtained over time, a determination can be made as to when a formation oil is being obtained as opposed to a mud filtrate. Further, as set forth in U.S. Pat. No. 5,331,156 to Hines, by making optical density (OD) measurements of the fluid stream at certain predetermined energies, oil and water fractions of a two-phase fluid stream may be quantified.
In situ gas quantification is described in U.S. Pat. No. 5,167,149, and U.S. Pat. No. 5,201,220 (both incorporated by reference herein), where a rough estimate of the quantity of gas present in the flow stream can be obtained by providing a gas detection module having a detector array which detects reflected light rays having certain angles of incidence.
Gas:Oil ratio (GOR) is an important property of fluids obtained from hydrocarbon wells and which is normally only measured at the surface. U.S. Pat. No. 5,939,717 (incorporated herein by reference) describes methods for determining GOR which include providing an OFA module which subjects formation fluids to NIR illumination and which provides a spectral measurement of peaks at about 6,000 cmxe2x88x921 and about 5,800 cmxe2x88x92. The methods include calculating a ratio of the amplitudes of the absorption peaks to determine GOR. Alternatively, the methods of calculating the ratio include referring to a database of spectra of hydrocarbons found in formation fluid and adjusting the amplitudes of the methane and oil peaks to account for the influences of other hydrocarbons on the spectrum of the formation fluid.
While GOR is in itself a useful measurement, the development of the measured GOR over time as fluids flow from the formation into the OFA flow line can be used to determine the degree of contamination of the formation fluids by oil-based mud filtrate or the like. Examples of this approach are found in U.S. Ser. No. 09/255,999, now U.S. Pat. No. 6,274,865 and U.S. Ser. No. 09/300,190, now U.S. Pat. No. 6,350,986 (both incorporated herein by reference).
The present invention seeks to provide improved methods for estimating GOR and associated measurements, methods for interpreting such measurements, and apparatus suitable for making such measurements.
A first aspect of the present invention provides a method of determining GOR comprising subjecting a fluid to spectroscopic analysis at a first wavelength sensitive to gas and a second wavelength sensitive to oil, determining a response matrix for the contribution of gas at the first and second wavelengths and the contribution of oil at the first and second wavelengths, determining a signal response vector and the two wavelengths, calculating a mass fraction vector from the response matrix and the signal response vector and using the mass fraction vector to determine GOR.
A second aspect of the invention provides apparatus for determining GOR which includes a spectroscopy module operating at least at a first wavelength sensitive to gas and a second wavelength sensitive to oil, means being provided to determine GOR from a mass fraction vector derived from a response matrix and a signal response vector.
A third aspect of the invention provides an method of compensating for temperature effects in spectroscopic measurements on formation fluids, comprising determining temperature dependency curves for source and measurement data, and analysing the fluid based on the measured response and the temperature dependency curves.
A fourth aspect of the invention provides a method for detecting gas in a flow line, comprising subjecting the fluids to spectroscopic measurements in the flow line at least at a wavelength sensitive to the presence of methane, and using the measured response to indicate the presence of gas.
A fifth aspect of the invention provides a method of detecting contaminants in the fluid in a flow line, comprising subjecting the fluids to spectroscopic measurements in the flow line at least at a wavelength sensitive to the presence of methane, and using the measured response to indicate the presence of contaminants.
A sixth aspect of the invention provides apparatus for analysing fluids downhole, comprising two spectroscopic analysis modules connected in series to a flow line, correlation of the outputs of the modules being used to calculate the flow rate of fluid in the flow line.